1. Field of the Invention
The present invention relates to a photosensitive insulating paste suitable for forming an insulating layer in a high-frequency circuit, and to a thick-film multi-layer circuit substrate containing the paste.
2. Description of the Related Art
As high-frequency electronic apparatuses, including cellular phone terminals and computers, have come to have higher density and higher signal speed, insulating materials used in high-frequency circuits or like elements of the apparatuses are required to have lower dielectric constants. Insulating materials are primarily used for forming an insulating layer that separates two or more electrodes or transmission lines in, for example, a high-frequency circuit substrate or an electronic component for a high-frequency circuit.
Such an insulating layer often contains micro-pores, i.e., so-called via holes, for electrically connecting electrodes and circuits such as those formed of transmission lines, which are disposed on the upper and lower surfaces of the layer. The insulating layer containing via holes is formed through the following procedure, for example. An insulating paste is applied onto a substrate by way of printing at a predetermined position by means of a technique such as screen printing, and the paste is dried. Subsequently, via holes are formed in the paste and the paste is fired to thereby form the insulating layer.
Meanwhile, use of a photosensitive insulating paste is also known as a technique for forming pores serving as via holes in an insulating layer. The technique uses an insulating paste containing a photosensitive resin such as a photosensitive polyimide resin, and micro-pores for via holes are formed in the paste by means of photolithography.
For example, Japanese Patent Application Laid-Open (kokai) Nos. 110466/1997 and 50811/1996 disclose a technique for forming micro-via holes. According to the technique, a glass material containing SiO2, Al2O3, B2O3, Bi2O3 and ZnO or a glass material containing Bi2O3, SiO2, B2O3, BaO and ZnO is dispersed in a photocurable organic vehicle to thereby obtain a photosensitive insulating paste, the paste is applied onto a substrate and micro-via holes are formed by means of photolithography in electronic elements for forming circuits which require multi-layer wiring. Examples of insulating materials having low dielectric constants and used for forming the above-described insulating layer include an insulating material containing a powder having a low thermal expansion coefficient, such as cordierite.
However, among the above-described techniques, the technique making use of screen printing may cause some problems, such as bleeding due to the viscosity of an insulating paste and poor printing resolution attributable to a printing plate. Therefore, an insulating layer containing micro-via holes having a diameter of, for example, less than 150 xcexcm is difficult to form, and thus, a circuit substrate and electronic components for constituting a circuit has not fully satisfied the demand for reduction in size and enhancement of density.
Meanwhile, the technique for forming an insulating layer through photolithography by use of a photosensitive insulating paste enables formation of micro-via holes having a diameter of, for example, less than 150 xcexcm. However, the thus-obtained insulating layer has poor durability at high temperature and low plating resistance to an acidic plating solution.
Particularly, glass materials disclosed in Japanese Patent Application Laid-Open (kokai) Nos. 110466/1997 and 50811/1996 are difficult to form into an insulating layer having high reliability when an Ag-type conductive material is used for forming transmission lines or electrodes adjacent to an insulating layer formed of the glass materials since the Ag-type conductive material is highly reactive with the insulating layer and Ag may disperse into the insulating layer. In addition, the glass materials may react with the organic binder to effect ionic cross-linking. For example, even when a glass powder is stabilized by use of triazole or a like compound, the viscosity of the photosensitive insulating paste may decrease with passage of time and reliable patterns may fail to be formed.
Meanwhile, a thick-film multi-layer circuit substrate containing an insulating paste or a photosensitive insulating paste may have camber and distortion caused by a mismatch between the thermal expansion coefficient of an alumina substrate or the like and that of the insulating layer. Particularly when an insulating layer predominantly composed of cordierite, which has a low thermal expansion coefficient, is formed on an alumina substrate, the substrate may have considerable camber and distortion caused by a large difference between the thermal expansion coefficients.
Also, when conductive patterns are formed adjacent to an insulating layer, the conductive material generally has a high thermal expansion coefficient of at least 14 ppm/xc2x0 C., resulting in a mismatch among the respective thermal expansion coefficients of the substrate material, the insulating layer, and the conductive patterns. In addition, depending on the circuit patterns, the produced circuit substrate may have considerable camber and distortion.
In view of the foregoing, there exists a demand for a photosensitive insulating paste which enables formation of micro-via holes, permits consistent film formation, and also permits arbitrary selection of thermal expansion coefficient.
Also, there exists a demand for a highly reliable thick-film multi-layer circuit substrate which has suppressed camber or distortion and which enables an increase in the density of conductive patterns.
Accordingly, in one aspect of the present invention, there is provided a photosensitive insulating paste comprising an insulating material containing a borosilicate glass powder, which material is dispersed in a photosensitive organic vehicle, wherein the borosilicate glass powder contains SiO2, B2O3 and K2O such that the compositional proportions by weight of the three components represented by (SiO2, B2O3, K2O) fall within a region formed by connecting points A (65, 35, 0), B (65, 25, 10), C (85, 5, 10) and D (85, 15, 0) in the ternary diagram shown in FIG. 1.
Preferably, the borosilicate glass powder has the compositional proportions by weight of the three components represented by (SiO2, B2O3, K2O) falling within a region formed by connecting points E (75, 25, 0), F (75, 20, 5), G (85, 10, 5) and D (85, 15, 0) in the ternary diagram shown in FIG. 1.
Preferably, the insulating material contains the borosilicate glass powder in an amount of at least about 60 wt.%.
Preferably, the borosilicate glass powder has a mean particle size of about 0.1-5 xcexcm.
When the ratio of surface area to weight is represented by SS (m2/g), the ratio of surface area to volume is represented by CS (m2/cc), and specific gravity is represented by xcfx81, the borosilicate glass powder preferably has a shape-smoothness index (xcfx81xc3x97SS/CS) of about 1.0-3.0.
Preferably, the insulating material additionally contains at least one species selected from the group consisting of a quartz powder, a vitreous silica powder and a high-silica glass powder.
Preferably, the insulating material also contains a low-melting glass. Preferably, the low-melting glass is a Bi2O3xe2x80x94B2O3xe2x80x94SiO2 glass.
Preferably, the photosensitive organic vehicle contains an organic binder, a photopolymerization initiator and a photocurable monomer, and the organic binder is an acrylic copolymer having a carboxyl group and an ethylenic unsaturated group in side chains of the copolymer. Preferably, the acrylic copolymer is an acrylic/modified-cellulosic copolymer. Preferably, the acrylic/modified-cellulosic copolymer has a compositional ratio of about 3/1-10/1.
In another aspect of the present invention, there is provided a thick-film multi-layer circuit substrate comprising an insulating substrate, an insulating layer, and a conductive layer, the insulating layer and the conductive layer being formed on the insulating substrate, wherein the insulating layer is formed through exposure, development and firing, after the above-described photosensitive insulating paste according to the present invention is applied thereto.
Preferably, the insulating layer is provided between a first conductive layer and a second conductive layer, each of the conductive layers being formed of an Ag-type conductive material, a Cu-type conductive material or an Au-type conductive material, wherein the insulating layer contains a via hole electrically connecting the first conductive layer with the second conductive layer.
Some effects obtainable from the present invention will be summarized hereinbelow.
In the photosensitive insulating paste according to the present invention, the insulating material contains a borosilicate glass powder having the predetermined compositional proportions by weight as shown in FIG. 1, and the borosilicate glass powder of the proportions contains a relatively large amount of an SiO2 component which has low reactivity with, for example, an organic binder. Therefore, the viscosity of the paste undergoes minimal changes with passage of time and a film can be formed in a consistent manner. As a result, micro-via holes of good shape can be formed with ease by means of photolithography. Moreover, the thermal expansion coefficient of the borosilicate glass powder can be freely set in accordance with the type of the substrate and the circuit pattern, since the thermal expansion coefficient can be varied with ease by suitably selecting the compositional proportions by weight of the borosilicate glass powder.
In the thick-film multi-layer circuit substrate according to the present invention, the insulating layer is formed of the photosensitive insulating paste of the present invention, and micro-via holes of good shape can be formed on the layer by means of photolithography. Thus, the densities of via holes and conductive patterns can be increased. In addition, the thermal expansion coefficient of the insulating layer can be suitably selected in accordance with the type of the substrate and the circuit pattern, and thus, a thick-film multi-layer circuit substrate having high reliability with suppressed camber and distortion can be obtained.